1. Field of the Invention
The present invention relates to adjusting a reverse rotation amount of a reverse rotation process in an injection molding machine having a check ring on the end of an injection screw, which is closed by reverse rotation of the screw.
2. Description of the Related Art
Heretofore, an injection molding machine having an injection mechanism, such as an in-line screw injection molding machine, has been known, which is equipped with a check ring mechanism on the end of a screw, for preventing backflow of a resin during injection thereof. In such a check ring mechanism, a mechanism is generally utilized in which a resin flow passage is closed and opened by means of a resin pressure difference that occurs in front of and behind the check ring.
A resin pellet, which is supplied to the inside of a cylinder from behind the screw, is melted by shear heat generated upon rotation of the screw, and by heat from a heater disposed on an exterior side of a cylinder into which the screw is inserted. The molten resin causes the resin pressure to rise behind the check ring, thereby generating a force that presses the check ring forward. When the check ring is pressed forward, the resin therebehind passes through a gap between the check ring and a reduced diameter portion, and flows in front of the check ring, thereby raising the pressure in the cylinder in front of the screw head.
When the pressure in front of the check ring exceeds a predetermined pressure (back pressure), the screw is pressed backward and the pressure in front of the check ring is reduced in pressure. Further, by rotating the screw, since the pressure behind the check ring becomes greater than the pressure in front of the check ring, the molten resin continues to be fed to the front of the check ring, and as a result, when the screw is retracted a predetermined amount, rotation of the screw is halted.
Next, an injection process is initiated. In order for the resin to be filled, the screw is advanced, and because the pressure of the resin collected in front of the screw head is raised, the check ring is retracted and comes into intimate contact with a check seat, thereby closing the resin passage, and a metered amount of resin is injected into and fills the interior of a mold. After initiation of injection, during the interval from the start of injection process and retraction of the check ring up to closing of the resin passage, the resin passage is kept open, and backflow of the resin occurs. Because of this fact, the timing for closing the resin passage fluctuates and the amount of resin to be filled also fluctuates, such that the molding process becomes unstable.
Consequently, as a method for performing injection while preventing backflow of the resin at the time of injection, a method is known in which a check ring is used, which closes the resin passage by rotating the screw in reverse (rotation in a direction reverse to the direction of screw rotation during the metering process). After completion of measuring and before injection is initiated, the screw is rotated in reverse to close the resin passage of the check ring. Thereafter, the screw is advanced and injection is performed.
For example, with the technique according to the check ring disclosed in Japanese Patent Application Laid-Open No. 3-45325 (Patent Document 1), a push metal is fixedly attached to the screw, and a first resin passage is formed by a groove provided in the axial direction in the push metal. Further, a check ring is attached in a non-fixed condition to the screw, and a second resin passage is formed by a groove provided in the axial direction in the check ring. A pin provided in the screw head engages with a circumferential groove of a fixed length, which is disposed in the check ring. Consequently, when the screw is rotated in a positive direction (the rotation direction of the screw during metering is designated as positive rotation), the push metal also rotates positively, and as a result, by engagement of the pin and the groove, the check ring is moved relatively with respect to the push metal by the length of the groove formed in the push metal, whereby the first resin flow passage and the second resin flow passage are brought into communication with each other. Thereafter, the communicative state of the resin flow passages is maintained by engagement of the pin with the end of the groove, and the check ring is rotated together with the screw. In addition, when the screw is rotated in reverse, by engagement of the pin and the groove, the check ring is moved relatively by the length of the groove with respect to the push metal, and as a result, the first resin flow passage and the second resin flow passage are not communicated with each other and they are sealed.
Further, with the technique according to the check ring disclosed in Japanese Patent Application Laid-Open No. 4-71817 (Patent Document 2), resin passages are closed by causing reverse rotation of a screw, which is roughly the same as that disclosed in Patent Document 1. A check ring is fitted rotatably onto a small diameter part of the screw end. Holes defining molten resin passages are formed in the axial direction in the check ring. Further, recesses are formed at the end of the screw main body and at the rear end of the screw head. A projection disposed on the check ring engages with a fixed length peripheral groove provided on a shaft on which the screw head is assembled, whereby the check ring is mounted onto the screw. Consequently, when the screw is rotated positively, through engagement of the projection and the groove, the check ring is moved relatively with respect to the screw by the length of the groove, and thereafter the check ring rotates integrally with the screw. By means of such relative movement, the molten resin passages of the check ring and the recesses at the rear end of the screw head are brought into positional agreement, and resin passages are formed that communicate the front portion with the rear portion of the check ring. In addition, by rotating the screw in reverse, the check ring is moved relatively with respect to the screw by the length of the groove through engagement of the projection and the groove, so that the resin passages of the check ring are made to face the portions other than the recesses at the end of the screw main body and the rear end of the screw head, thereby closing the resin passages.
Further, with the technique according to the check ring disclosed in Japanese Patent Application Laid-Open No. 9-174629 (Patent Document 3), a check ring is disclosed in which, by positively rotating a screw that is roughly the same as those disclosed in Patent Documents 1 and 2, a ring is caused to rotate so that a resin passage is opened. Further, by rotating the screw in reverse, the resin passage is closed.
Furthermore, with the technique according to the check ring disclosed in Japanese Examined Patent Publication No. 47-8380 (Patent Document 4), an annular body is fitted rotatably between a screw head and the end of a screw shaft. An inclined surface is formed on the rear end surface of the screw head, and another inclined surface that contacts the inclined surface of the screw head is formed on the annular body. During metering, by positively rotating the screw, a resin passage is opened between the annular body and the screw shaft, such that front and rear portions of the annular body are communicated with each other. When the screw is rotated in reverse, the annular body is moved in an axial direction by the inclined surfaces of the screw head and the annular body. As a result, a valve contact face at the rear end of the annular body and a contact face at an end of the screw shaft are joined, whereby the resin passage is closed.
Further, in the techniques according to the check rings disclosed in Japanese Patent Application Laid-Open No. 60-201921 (Patent Document 5) and Japanese Patent Application Laid-Open No. 62-19423 (Patent Document 6), an inclined surface is formed on the rear end surface of a screw head so that it is engaged by a projection of a check ring. When the screw is rotated in reverse, the check ring is pressed by the inclined surface of the screw head, and as a result, the check ring is retracted and the resin passage is closed.
If the screw is advanced and injection is performed while the check ring is not closed and the resin passage is in an open state, a backflow of the resin is generated. Although a reverse rotational torque is imposed on the screw by the resin backflow, it is known that the reverse rotational torque stops acting on the screw when the check ring is closed during forward movement of the screw. For example, in Japanese Patent Application Laid-Open No. 2004-216808 (Patent Document 7), when the screw is made rotatable and injection is initiated, at first, the screw is rotated in reverse by backflow of the resin, however, when the check ring is closed, the reverse rotational torque stops acting on the screw and reverse rotation of the screw is halted. Consequently, closing of the check ring is detected by detecting the stoppage of the reverse rotation of the screw.
Compared with the case of a check ring in which the resin passage is closed as a result of a difference in resin pressure occurring in front of and behind the check ring at the time the screw is advanced, in the case of a check ring in which the resin passage is closed by rotating the screw in a direction opposite to the rotation direction at the time of metering, the resin passage can be closed more reliably. Also, because resin backflow generated at the time of injection is prevented, the amount of resin that fills the mold is made uniform and stable molding is enabled.
For this reason, when a check ring which closes the resin passage by rotating the screw in reverse is used, it is desirable to obtain an appropriate reverse rotation amount of the screw that closes the resin passage.
When the screw is rotated in reverse, although the screw head is rotated in reverse at the same speed as the screw, at this time, due to an effect of resin viscosity, the check ring also tends to rotate in reverse at a speed together with the screw head. As a result thereof, when the screw is rotated in reverse at a predetermined amount, the amount at which the check ring is rotated with respect to the screw head becomes smaller than the reverse rotation amount of the screw. Accordingly, when the screw is rotated in reverse for closing the flow passage, a screw reverse rotation amount is required, which is larger than the reverse rotation amount obtained from the design value of the check ring closing mechanism.
However, because the relative speed at which the check ring rotates with respect to the screw head fluctuates based on the viscosity of the resin and the like, the reverse rotation amount required for closing also varies according to the viscosity of the resin. For this reason, when the resin or other design conditions are changed, the reverse rotation amount, which is required for closing the resin passage, also changes.
With the check rings disclosed in Patent Documents 1 to 3, the resin passage is closed solely by reverse rotation of the screw. Accordingly, when the screw is rotated in reverse prior to start of injection and thereafter the injection process is carried out, in the event that the reverse rotation amount was insufficient, the resin passage remains open, and therefore, resin backflow occurs during injection, which is undesirable. On the other hand, in the case where the reverse rotation amount is excessive, although the resin passage is closed, the resin flows backward behind the screw more than necessary during reverse rotation, resulting in inconveniences such as an increase in metering time and long resin residing time.
On the other hand, with the check rings disclosed in Patent Documents 4 to 6, the resin passage can be closed by reverse rotation of the screw, and even in the case where the screw rotation amount is insufficient and the resin passage is not closed, the check ring can be caused to retract even by the resin pressure at the time of injection so as to close the resin passage. However, in the case where a step of rotating the screw in reverse to close the resin passage before start of the injection process is carried out as one step of the molding process, it is preferable for the resin passage to be reliably closed by reverse rotation of the screw. In addition, as described above, the screw reverse rotation amount for closing the resin passage of the check ring fluctuates due to the viscosity of the resin and the like. Therefore, it is desirable to determine a reverse rotation amount required for closing the resin passage, for reliably closing the resin passage in spite of fluctuations in viscosity or the like, and moreover, for suppressing an increase in the cycle time and minimizes residing time of resin.
In the conventional art, when using a check ring for closing a resin passage by reverse rotation of the screw, the fact that the amount of reverse rotation of the screw required for closing the resin passage varies according to molding conditions is disregarded, and a method for determining an optimal reverse rotation amount which is suitable to the molding condition concerned has not been known yet.